1. Field of Invention
The present invention relates to an imaging device that uses a MOS-type solid state image sensor, to an imaging method, and to an image sensor.
2. Description of Related Art
Solid state imaging devices (image sensors) are used to convert light from a subject being recorded to an electric signal in imaging devices such as digital cameras and video cameras that are used to capture and store pictures. Solid-state image sensors include primarily CCD image sensors and MOS image sensors, although CCD image sensors are predominant today. This is because CCD image sensors afford better performance including a wider dynamic range and better real-time imaging performance. CCD image sensors suffer from high power consumption and requiring multiple supply voltages, however, and future improvement in these characteristics is believed to be difficult.
MOS image sensors have a narrow dynamic range and weak noise resistance when compared with CCD image sensors, but have improved with regard to these deficiencies as a result of recent research. CMOS image sensors offer numerous benefits that are not afforded by CCD image sensors, including low power consumption and the ability to freely control the order in which pixels are read. Furthermore, because CMOS devices can also be manufactured using the same equipment and materials used to produce LSI devices, the equipment that is currently used to manufacture LSI devices can be used as is to manufacture CMOS image sensors, thus helping to reduce the production cost. CMOS image sensors thus offer numerous benefits and have recently attracted wide interest.
Solid-state image sensors are used in such devices as digital still cameras, video cameras, and cell phones, and produce images by the process described below when recording pictures and video.
(a) Electric signals produced by the CCD or MOS sensor are read one pixel at a time line by line from the first pixel in the sensor, and temporarily stored to relatively inexpensive SDRAM (synchronous DRAM) or other memory device.
(b) When reading and writing one line to SDRAM is completed, line 2, line 3, and so forth are sequentially read to write one frame to SDRAM.
(c) The signals are then read from SDRAM and passed through a signal processing operation such as a zoom process to enlarge or reduce the image, and the data resulting from this operation is temporarily stored to SDRAM again.
(d) The data from the signal processing operation is then read from SDRAM, compressed, and converted to a compressed data format such as JPEG that is suitable for recording, and the compressed data is stored to SDRAM again.
(e) The compressed data is then read at high speed from SDRAM by DMA (direct memory access) control, for example, and output to semipermanent external storage.
The number of pixels in the images that are captured by digital still cameras (DSC) continues to grow. Digital still cameras with resolution exceeding 16 million pixels are now available, and even fairly low cost digital cameras offer resolution of three million to five million pixels.
Video cameras capture and consecutively present from 30 frames to 60 frames per second to output a moving picture image, and the image resolution of each frame ranges from 640×480=307,200 pixels in a VGA image to 1920×1080=2,073,600 pixels in a high definition (HD) image.
While the image sensors used in video cameras only require a resolution of approximately 300,000 pixels and the ability to capture at least 30 frames/second in order to record a VGA size image, demand is also strong for consumer-grade video cameras that can also take still pictures with a resolution of 2 to 3 megapixels. As a result, video cameras are commonly manufactured using high resolution image sensors and are designed to switch the imaging operation to record video and to record still pictures.    Reference patent: U.S. Patent Application Publication No. 2003/0077074 (corresponding to Japanese Laid-open Patent Publication No. 2002-251819)
In order to record video at a frame rate of 1/30 second, the image data for one frame must be read out in 1/30 second or less. It is, however, difficult to read the pixel data for the several megapixels constituting a high resolution still picture in 1/30 second or less.